Radiant electric heaters are well known comprising a base of thermal and electrical insulation material, such as microporous insulation material, having supported thereon at least one electric heating element such as of bare wire or ribbon form. The base of insulation material can be provided in a dish-like support, such as of metal. The base has a flat surface and/or may be formed in discontinuous sections. A wall of thermal insulation material is provided around the periphery of the heater and an upper surface of the wall is arranged to contact the underside of a glass-ceramic cooking plate.
It is usual to provide a rod-like temperature-responsive device extending at least partly across the heater, between the heating element or elements and the glass-ceramic plate and arranged substantially parallel to the glass-ceramic plate. Such temperature-responsive device is arranged to de-energise the heater at a predetermined sensed temperature to prevent overheating of the glass-ceramic plate.
Increasing use is being made of rod-like temperature-responsive devices which extend only partly across the heater and which have an end located in a middle region of the heater. Such temperature-responsive devices may comprise a metal rod of relatively high thermal expansion inside a tube of relatively low thermal expansion. The tube is generally of electrical insulating material, such as quartz or fused silica, or a ceramic such as cordierite. The rod and tube are mechanically connected at the end which is located in the middle region of the heater and a metal component is generally exposed at this end location.
The heating element or elements is or are normally supported on a flat surface of the base of insulation material, for example in a plane parallel to that of the glass-ceramic plate.
With this arrangement the metal component at the end of the rod-like temperature-responsive device may be in close proximity to the one or more heating elements and this is particularly so when the element or elements is or are provided directly underlying the end of the device such as, for example, when the element or elements is or are provided in spiral form from the periphery of the heater to the middle of the heater.
A problem arises with such an arrangement in that electrical safety legislation sets down limits for the minimum distance between the heating element or elements, that may be of live electrical potential, and neighbouring metal components, that are of zero electrical potential or connected to earth, such as the metal component at the end of the rod-like temperature-responsive device. Furthermore, heater manufacturers are under pressure to provide heaters which are compact and with as low a profile as possible, such that the distance between the heating element and the glass-ceramic is as small as possible.